Experiments have been conducted to determine the in vitro permeability of rabbit aortic media to albumin and water. Results to data indicate that the media has convective and diffusive permeability properties equivalent to a membrane consisting of 70-80 A circular pores. The hydraulic permeability is such that greater than 90 percent of the albumin flux is attributable to bulk fluid flow when delta P equals 100 mm Hg. These findings suggest that considerable sieving occurs as macromolecules permeate the aortic wall. The tissue's permeability to low-density lipoproteins will be studied next.